Solid golf balls are typically classified into one-piece golf ball and two-piece solid golf ball, the two-piece solid golf ball is generally approved or employed by amateur golfers. The two-piece solid golf ball is composed of a solid core of molded rubber material and a cover typically formed from ionomer resin. The two-piece solid golf ball exhibits long flight distance, but shows poor shot feel when hitting. The two piece solid golf ball is so hard that it is difficult to apply spin on the two-piece solid golf ball and that it is difficult to control it when hitting by an iron club or at approach shot.
In order to improve the shot feel and controllability of the two-piece solid golf ball, a multi-piece solid golf ball comprising a core having two or more layers has been proposed (Japanese Patent Kokai Publication No. 194760/1984). The solid core is generally formed from a rubber composition, and it is difficult to form the rubber composition in two or more layers and in a concentric sphere. Therefore, it is not suitable to mass production.
In order to solve the problem, a multi-layer core comprising of a center (an inner core) formed from rubber composition and an outer core formed on the inner core from thermoplastic resin has been proposed (Japanese Patent Kokai Publication Nos. 244174/1992 and 142228/1994). When using thermoplastic resin, the outer core can be formed on the center of the core uniformly and in a concentric sphere, but the core has poor impact resistance when hitting repeatedly and poor form stability because of using thermoplastic resin. In some cases, the use of the thermoplastic resin has a problem in adhesion properties between ionomer resin which has been typically used for the cover of golf balls and the thermoplastic resin used for the outer core. In this case, energy loss occurs in the interface between the outer core and the cover, which leads to the reduction of flight distance.